1. Field of the Invention
This invention relates to a coating composition which provides under irradiation with ultraviolet rays in air a crosslinked and cured coating film excellent in the abrasion resistance, flexibility, heat resistance, solvent resistance, adhesion to the substrate and weatherability, and also to a process for preparing synthetic resin shaped articles excellent in the abrasion resistance and weatherability by using this coating composition.
2. Description of the Prior Art
Synthetic resin shaped articles prepared from polymethyl methacrylate resins, polycarbonate resins, and other similar resins have a light weight and are excellent in their impact resistance, and these synthetic resins are cheap and can easily be molded. Because of these various advantages, these synthetic resin shaped articles have been used in various fields and new uses of these synthetic resin shaped articles have been developed in various fields. However, these synthetic resin shaped articles are poor in the abrasion resistance, and therefore, the surfaces are readily damaged by contact with a hard article or abrasion or scratching. Damage caused on the surfaces of these shaped articles lower the commercial value thereof drastically and shorten the service lives of these shaped articles. Therefore, it has eagerly been desired to improve the abrasion resistance of the surface in these synthetic resin shaped articles. Shaped articles having an improved abrasion resistance, which are prepared by coating the above-mentioned synthetic resin shaped article with a coating material, are often used outdoors. For example, they are used as windshields of air planes, outdoor signboards, traffic control signs and automobile parts, and therefore, a high weatherability is also eagerly desired in these shaped articles.
Various methods have heretofore been proposed as means for eliminating these defects of the above-mentioned synthetic resin shaped articles. For example, there can be mentioned a method in which a coating material comprised of a resin composition comprising a silicone or melamine type resin is coated on the surface of a synthetic resin shaped article and then, the coating material is heated to be thereby condensed to yield a crosslinked coating film having an improved abrasion resistance on the synthetic resin shaped article, and a method in which a polyfunctional monomer containing at least two polymerizable groups in the molecule is coated on the surface of a synthetic resin shaped article and then, the coated shaped article is irradiated with active energy rays to form a crosslinked and cured coating film on the surface of the synthetic resin shaped article, whereby a synthetic resin shaped article having an excellent abrasion resistance is obtained (see U.S. Pat. Nos. 3,518,341 and 4,041,120 and Japanese Laid-open Patent Applications Nos. 102,936/78, 104,638/78 and 97,633/79).
Indeed, the abrasion resistance of the surface of synthetic resin shaped articles can be improved by these methods. However, these methods are not completely satisfactory in some points. For example, the products obtained according to the former method are poor in durability to thermal shocks and hot water, and in chemical resistance. Furthermore, although the products obtained according to the latter method are satisfactory in durability and chemical resistance, they are poor in weatherability and must be produced in an atmosphere of an inert gas which is costly.
Under such background, we did research with a view to developing a synthetic resin shaped article excellent in not only abrasion resistance but also weatherability, and we have now found that when a coating composition comprising at specific ratios a monomer mixture comprising a specific polyfunctional acrylate and a specific monofunctional acrylate in a specific proportion, and a photosensitizer optionally with a specified light stabilizer and a specified silicone type surface active agent is coated on a synthetic resin shaped article and is then cured, a synthetic resin shaped article having excellent abrasion resistance and weatherability can be advantageously obtained.